Development of CMAQ Boundary Conditions

One of the necessary inputs for performing CMAQ simulations is the boundary condition (BC). The past CMAQ regional photochemical modeling covering episodes relied on the use of ‘clean’ BCs. Recently, it has been recognized that seasonal and annual simulations, the appropriate BCs can be developed fr om global models (EPA, 2004). With the development and progress of global chemical transport models such as GEOS-CHEM of Harvard University ( http://www-as.harvard.edu/chemistry/trop/geos/index.html ) and MOZART of National Center for Atmospheric Research (NCAR) ( http://www.acd.ucar.edu/science/gctm/mozart/index.php ), it is possible to generate BCs for CMAQ through appropriate interpolat ion of the global model outputs.

In this note we describe the developmen t of the BCs for the OTC Modeling Committee application of CMAQ at 36 km grid spacing and perform a limited comparison with the BCs reported for Visibility Improvement State and Tribal Association of the Southeast’s (VISTA) modeling domain. GEOS-CHEM si mulation data for 2002 were obtained through the efforts of the No rtheast Consortium of Air Us e Management (NESCAUM) in

December 2004. The GEOS-CHEM simulation data was at the spatial resolution of 4  ̊ by

5  ̊ in the horizontal and 20 layers in the ve rtical extending from su rface to about 100 mb. The model provides information for about 50 chemical gas and particulate matter chemical species. We utilized and modified version of the GEOS-CHEM to CMAQ interface program developed by Prof. Daewon By un of Huston University so as to match the OTC 36 km modeling domain. To provide an added level of confidence in the development of the BCs, a comparison is performed with the BCs reported for the VISTAs 36 km modeling domain. It should be noted that the two domains differ in their definition with the OTC domain being smalle r horizontally and has more layers in the vertical when compared with the VISTA domain.

Spatial patterns of Ozone and SO 4

Figure 1 displays ozone concentrations in layer 1 along the boundaries for OTC and VISTA domains at hour 15 on August 13, 2002. Although the OTC domain is slightly smaller than VISTA domain, both exhibit sim ilar maximum concentrat ion level of about 70 ppb. The OTC domain had slightly higher p eak ozone (168 ppb) at the top of the domain along the boundaries than VISTA (s ee Figure 2), which has a maximum of 139 ppb. This is not unexpected since the top laye r of the OTC domain is higher than VISTA domain. Figure 3 shows similar displays for SO 4 at the boundary at th e lowest layer of

the OTC and VISTA domains.

Averaged Ozone and SO 4 concentrations at the boundaries of the modeling domain

Table 1 lists the averaged ozone concentrations at surface and for the whole boundary layer, (OTC domain with 22 layers, and VI STA domain with 19 layers), at each one of

the four boundaries. In both cases the concentr ations exhibit similar pattern, with higher

concentrations at south and east boundaries and low concentr ation in the north and west

boundaries. Table 2 is similar to Table 1, except it lists SO 4 concentrations. The higherSO 4 boundary conditions at east direction may be due to the outflow from the US

continent.

Averaged Ozone and SO 4 BC vertical profiles from each of the domain

Figure 4 displays the average vertical profil e of ozone concentrations at each of four boundaries. As to be expected, GEOS-CHEM yields increasing oz one concentrations with increasing altitude and this is reflected in Figure 4. Figure 5 displays the average vertical profile of SO 4 concentration at each of the four boundaries. The high SO 4 in the East direction may be due to the outflow from the US continent.

References

EPA (2004): Use of GEOS-CHEM for CMAQ Boundary Conditions, http://www.epa.gov/air/interstateairquali ty/pdfs/GEOSCHEMforC MAQ_Description.pdfTable 1: The average ozone concentrations at each one of the four boundaries for

the OTC and VISTA 36 km domains

Boundary OTC (22 layer averaged) VISTA (19 layer averaged) OTC (surface) VISTA (surface)

South 43.9 ppb 35.6 ppb 36.8 ppb 31.6 ppb

East 52.2 ppb 46.4 ppb 40.5 ppb 40.7 ppb

North 34.1 ppb 21.9 ppb 13.7 ppb 13.5 ppb

West 40.0 ppb 32.9 ppb 25.8 ppb 24.9 ppb

Table 2: The average SO 4 concentrations at each one of the four boundaries for the

OTC and VISTA 36 km domains

Boundary OTC (22 layer averaged) VISTA (19 layer averaged) OTC (surface) VISTA (surface)

South 0.54 0.51 0.70 0.56

East 1.50 1.67 2.04 1.88

North 0.45 0.54 0.63 0.64

West 0.42 0.47 0.53 0.53Figure 1: The lowest layer of ozone BC for OTC and VISTA 36 Km domain

Figure 2: The top layer ozone BC for OTC and VISTA 36 Km domainFigure 3: The lowest layer SO4 BC for OTC and VISTA 36 Km domain

Figure 4: Averaged vertical profil es from each direction of ozone BCFigure 5: Averaged vertical profil es from each direction of SO4 BC